Peptides have recently attracted attention as a drug candidate or research tool. There have been various attempts to develop a peptide library and screen peptides having affinity or physiological activity for a target substance.
Patellamide produced by Prochloron didemni, that is, endozoic algae of sea squirt is a low molecular cyclic peptide which is presumed to have various physiological activities. It is biosynthesized via a unique pathway with products of a pat gene cluster consisting of patA to patG. The pat gene cluster and biosynthesis pathway of it are schematically shown in FIG. 10.
In this biosynthesis, PatE peptide which is a patE gene product becomes a precursor. Since the patE gene has a hypervariable region (cassette region), the product of it constructs a natural combinatorial library.
The PatE peptide has, on both sides of the cassette region thereof, a recognition sequence by a post-translational modifying enzyme. Enzymes which serve as the post-translational modifying enzyme are PatA, PatD, and PatG. PatD introduces an azoline skeleton into Cys, Ser, and Thr in the cassette of PatE and converts Cys into a thiazoline structure and Ser and Thr into an oxazoline structure.
PatA cleaves the N-terminal recognition sequence of the cassette region of the PatE.
PatG is composed of two domains. An N-terminal oxidase domain converts a thiazoline structure introduced by PatD into a thiazole structure. A C-terminal peptidase domain macrocyclizes, while cleaving a C-terminal recognition sequence of the cassette region of PatE.
The present inventors have already found that the cassette region of PatE is modified by PatD even when the sequence of it is changed to a variety of sequences different from a natural one and have confirmed that library of azoline-structure-containing peptides much more rich in variety than that of natural PatE products can be produced using PatD (Patent Document 1).
On the other hand, peptides having an azole structure are expected to have properties such as higher peptidase resistance than those having an amide bond, excellent binding ability to a target due to a fixed structure, high affinity for metals or nucleic acids because they are converted into aromatic compounds having a π electronic system, and high membrane permeability due to loss of a hydrogen atom of an amide bond serving as a hydrogen bond donor. As a natural physiologically active peptide having azole as a backbone structure, known are patellamideD having inhibitory activity against multi-drug resistance of a leukemia cell strain and Telomestatin having telomerase inhibitory activity.
If a diverse library of azole-structure-containing peptides can be obtained, therefore, it is possible to enhance the possibility of selecting a peptide having binding ability or physiological activity for various target substances.